Recently, an organic EL display and an FED display have been being studied and developed. Particularly, the organic EL display attracts attentions as a display used in a portable device, such as a cellular phone and PDA (Personal Digital Assistants), which can emit light at a lower voltage and with lower power consumption.
As to the organic EL display, a simple matrix type was introduced into the market, but an active matrix type will be mainly sold in the future. An organic EL active element can be realized by using an amorphous silicon TFT, but there is a tendency to use a smaller TFT, such as a monocrystal silicon TFT, a polysilicon TFT, and a CG (Continuous Grain) silicon TFT, which can be formed with a driving circuit at the same time and can drive the organic EL (that is, mobility of the TFT is high). Particularly, a low temperature polysilicon TFT and a CG silicon TFT which can be formed on a glass substrate are preferably used as materials for a direct view display.
As referred to in “Active Matrix Addressing of Polymer Light Emitting Diode Using Low Temperature Poly Silicon TFTs”, AM-LCD 2000 pp 249-252 (hereinafter, referred to as Document 1) and the like, a pixel circuit of the active matrix type organic EL using the low temperature polysilicon or the CG silicon basically includes two TFT elements Qa and Qb, a capacitor Ca, and an organic EL element E1a as shown in FIG. 13.
That is, the driving TFT element Qb is disposed in series with the organic EL element ELa between a power source wiring Vref and a power source terminal Vcom, and the capacitor Ca is disposed between a gate terminal and a source terminal of the driving TFT element Qb, and the source terminal is connected to the power source wiring Vref. Further, a gate terminal of the selecting TFT element Qa is connected to a gate wiring Gi, and a source/drain terminal is disposed so as to connect a source wiring Sj to the gate terminal of the driving TFT element Qb. A voltage is inputted from the source wiring Sj to the capacitor Ca under such condition that the selecting TFT element Qa conducts (ON state). As a result, conductance of the driving TFT element Qb is controlled, so that a current flowing to the organic EL element ELa is controlled, thereby controlling luminance of the pixel. Thereafter, a potential of the capacitor Ca is maintained under such condition that the selecting TFT element Qa does not conduct (OFF state), so that the conduction state of the driving TFT element Qb is maintained, thereby keeping the luminance of the pixel.
The luminance of the organic EL element is in proportion to a value of the current flowing to the organic EL element, so that the foregoing arrangement raises such a problem that: when an applied voltage/current property of the organic EL element ELa varies, the value of the current flowing to the organic EL element ELa varies.
FIG. 14 shows a pixel circuit arrangement shown in “Active Matrix PolyLED Displays”, IDW 00 pp 235-238 (hereinafter, referred to as Document 2). In the circuit arrangement shown in FIG. 14, a switching TFT element Qc is disposed between the driving TFT element Qb and the organic EL element ELa, and the selecting TFT element Qa is disposed between (i) a connection point of the driving TFT element Qb and the switching TFT element Qc and (ii) the source wiring Sj, and a switching TFT element Qd is disposed between the selecting TFT element Qa and the capacitor Ca. A gate terminal of the selecting TFT element Qa and gate terminals of the switching TFT elements Qc and Qd are connected to a gate wiring Gi.
In this arrangement, the selecting TFT element Qa and the switching TFT element Qd are turned ON under such condition that the switching TFT element Qc is turned OFF, so that a current flows from the power source wiring Vref to the source wiring Sj. An amount of the current is controlled by a current source of a source driving circuit (not shown), so that a gate voltage of the driving TFT element Qb is set so that a current whose amount has been determined by the source driving circuit flows to the driving TFT element Qb regardless of a threshold value voltage/mobility of the driving TFT element Qb. Further, the switching TFT element Qc is turned ON under such condition that the selecting TFT element Qa and the switching TFT element Qd are turned OFF, so that the capacitor Ca's potential at this time is maintained, thereby controlling the driving TFT element Qb so as to flow a predetermined amount of the current to the organic EL element ELa.
Further, FIG. 15 shows a pixel circuit arrangement shown in Japanese National Publication of Translated Patent No. 514320/2002 (Tokuhyo 2002-514320)(Publication date: May 14, 2002) (International Publication Number: WO 98/48403) (hereinafter, referred to as Document 3). In the circuit arrangement shown in FIG. 15, a switching TFT element Qg is disposed between the driving TFT element Qb and the power source wiring Vref, and a switching TFT element Qf is disposed between the driving TFT element Qb and the source wiring Sj, and a selecting TFT element Qe is disposed between the organic EL element ELa and the capacitor Ca. Gate terminals of the switching TFT elements Qf and Qg and a gate terminal of the selecting TFT element Qe are connected to the gate wiring Gi.
In this arrangement, the selecting TFT element Qe and the switching TFT element Qf are turned ON under such condition that the switching TFT element Qg is turned OFF, so that a current flows from the source wiring Sj to the organic EL element ELa. An mount of the current is controlled by a current driving circuit Pj of the source driving circuit (not shown), so that a gate terminal voltage of the driving TFT element Qb is set so that a current whose amount has been determined by the source driving circuit flows to the driving TFT element Qb regardless of a threshold value voltage/mobility of the driving TFT element Qb. Further, the switching TFT element Qg is turned ON under such condition that the switching TFT element Qf and the selecting TFT element Qe are turned OFF, so that the capacitor Ca's potential at this time is maintained, thereby controlling the driving TFT element Qb so as to allow a predetermined amount of the current to flow to the organic EL element ELa.
Note that, an arrangement of the CG silicon TFT is mentioned in “4.0-in. TFT-OLED Displays and a Novel Digital Driving Method (Semiconductor Energy Research Institute)” (hereinafter, referred to as Document 4) of SID'00 Digest pp. 924-927, and the like. Further, the CG silicon TFT process is mentioned in “Continuous Grain Silicon Technology and Its Applications for Active Matrix Display (Semiconductor Energy Research Institute)” (hereinafter, referred to as Document 5) of AM-LCD 2000 pp. 25-28, and the like. Further, the arrangement of the organic EL element is mentioned in “Polymer Light-Emitting Diodes for use in Flat Panel Display” (hereinafter, referred to as Document 6) of AM-LCD '01 pp 211-214, and the like.
However, in Documents 2 and 3, the driving TFT element Qb which functions as an active element for driving the organic EL element ELa by supplying a current having a predetermined value via the source wiring Sj during a selection period is arranged so that a gate terminal potential of the driving TFT element Qb is set, so that a value of a current flowing to the organic EL element ELa is determined on the basis of the current whose value has been determined. Thus, even when the applied voltage/current property of the organic EL element ELa varies, the value of the current flowing to the organic EL element ELa does not vary, which brings about such advantage that the luminance hardly varies.
However, the pixel circuit arrangement in Documents 2 and 3 is a 4-TFT pixel circuit arrangement in which a single organic EL element requires a capacitor, four TFT elements, a power source wiring, a source wiring, and a gate wiring. Thus, in the 4-TFT pixel circuit arrangement, an area occupied by the capacitor and the TFT elements is increased, so that an area occupied by a transparent electrode such as ITO for forming the organic EL element (that is, an anode area) is reduced. Particularly, minimum values of a TFT element size and a wiring width are determined on the basis of a process rule. Thus, even when the pixel size is reduced, it is impossible to make the TFT element size and the wiring width smaller.
Therefore, in a case of manufacturing a high definition panel of not less than 100 ppi, the 4-TFT pixel circuit arrangement shown in FIG. 14 or FIG. 15 causes the area occupied by the transparent electrode to be not more than half of the area of a 2-TFT pixel circuit arrangement shown in FIG. 13.
Further, a power source voltage preferably used to obtain a predetermined luminance varies depending on respective dots of RGB, so that it is desirable to prepare the power source wirings Vref different from each other corresponding to respective colors of RGB. In this case, the respective colors of RGB are disposed along the power source wirings Vref, and as shown in FIG. 16, a pixel circuit Aij is divided into three portions by the power source wirings Vref, so that the dots of RGB are formed. However, also the source wirings Sj are formed in parallel to the power source wirings Vref, so that wirings which exist in the pixel circuit Aij include: three power source wirings Vref, three source wirings Sj, and a gate wiring Gi.
As a result, a display device having the pixel circuit shown in FIG. 14 or FIG. 15 is such that: as shown in FIG. 16, besides a TFT area 7 and an area of the gate wiring Gi, there is a pixel area (RGB dots 9, 10, and 11 constitute a single pixel) which cannot be used to emit light due to the source wirings Sj, and the pixel area is so large that: pixel length×(source wiring width Y [μm]+process blank P [μm]×3. Here, pixel length=length of dots of RGB=width×[μm] of each dot of RGB×3. As a result, the foregoing arrangement raises such a problem that: an ITO area 8, that is, an area for forming the transparent electrode is made extremely small.